Answer:
Air
Step-by-step explanation:
Air.
Why? Because air has a much lower specific heat capacity (Heat capacity - Wikipedia) per unit mass than water. Also, air is dispersed as a gas so that in any given volume the mass of air that would fill it is only a fraction of the mass of water that would fill the same volume. Take these two factors into consideration and you see that the amount of energy it takes to raise the temperature of a volume of air by one degree is minuscule compared to the amount of energy it takes to heat the same volume of water by one degree.
The one thing that is more challenging about air is actually getting the energy into it. Water is densely packed, whereas air molecules are much more widely separated, so that presents a heat conductivity challenge. However, if we’re talking about equipment specialized for heating air (i.e. a fan forcing air over heated coils) versus equipment specialized for heating water (flat plate, coils, pumps or no pumps, whatever), energy added at the same rate to each will heat an equivalent volume of air to a greater extent (i.e. raising temperature more quickly) that it will water.